WASHINGTON -- The Islamic State poses a greater threat to the United States than al Qaeda did leading up to September 11, 2001, top brass at the Pentagon acknowledged on Thursday, discussing the crisis fast unfolding in eastern Syria and northern Iraq.

"This is beyond anything that we've seen," Secretary of Defense Chuck Hagel said, speaking to journalists with Chairman of the Joint Chiefs of Staff Martin Dempsey. "We must prepare for everything."

Hagel said that the "military prowess" of the Islamic State, coupled with its deep sources of financing, poses an unprecedented threat to the United States. The secretary was asked whether the group has a comparable operation capacity as Al Qaeda had in 2001.

General Dempsey added that, should the Islamic State succeed in forming a pseudo-state in the regions of Iraq and the Levant, the world would then be faced with an historic security challenge.

Both men said that US air strikes in Iraq thus far had successfully halted the progress of the group. Hagel said that US President Barack Obama had been presented with the option of striking Islamic State targets in Syria, as well, and that those strikes are continuously under review.

But earlier in the day, US officials said that the US military will not broaden its campaign against the Islamic State in either Iraq or Syria after it killed American journalist James Foley. Obama spoke sharply against the group on Wednesday, vowing a "relentless" US response to Foley's murder.

US officials said that, despite Foley's public beheading and the threatening of another journalist, Steven Sotloff, should strikes continue, the president has no plans to significantly increase air strikes against Islamic State fighters.

"From a military perspective, I don't think this is going to change anything," one US official said. "The military objective never was to degrade ISIL... it was to protect US personnel and facilities."

The US military will not broaden its campaign against the Islamic State in either Iraq or Syria after it killed American journalist James Foley, officials said on Thursday, after Obama spoke sharply against the group, and of a “relentless” response to Foley’s murder.

Last week, however, State Department deputy spokeswoman Marie Harf said the US focus at the moment was to hurt the Sunni terrorist group throughout northern Iraq.

“We’re focused on pushing them back from this territory,” Harf said.

Foley’s death was captured on video by the group, paired with a message that warned of more American deaths unless the US stopped attacking its assets. The American military has struck more than 90 Islamic State targets thus far around Arbil, an oil-boom town, Mount Sinjar, a refuge for religious minorities, and Mosul Dam, a key piece of infrastructure, all in northern Iraq.

Foley’s family received an email a week before the video was released, warning of his pending death and demanding a ransom of the US government in the amount of $132.5 million. The US government has a longstanding policy against paying ransom to terrorists, unlike its European partners, which have paid the Islamic State for the return of their citizens in the past several months.

“We do not make concessions to terrorists,” Harf reiterated on Thursday, saying that the US refuses to fund terrorism or to encourage more kidnappings.

The US Justice Department has opened an investigation into Foley’s murder, Attorney- General Eric Holder acknowledged on Thursday. Foley, who went missing while on assignment for news website GlobalPost in Syria in November 2012, was 40 years old at his death.

The Pentagon continued to conduct strikes on Islamic State targets throughout northern Iraq on Wednesday and Thursday nights, after the video was released and Obama had been briefed. More than two dozen attacks were conducted after the terrorist group released the video to the public.

Sotloff’s life rests on whether or not the president continues to authorize air strikes, the group said.

“First of all, there is no justification for these kind of barbaric acts, period,” Harf told reporters on Wednesday.

“Second, we don’t make concessions to terrorists,” she continued. “The president was clear we’re going to keep doing what we’re doing. And I would also note that, as I said earlier, ISIL has been willing to kill and rape and enslave anyone who gets in their way, regardless of what country they’re from, regardless of the policies of that country.”

Harf could not confirm that either Obama or US Secretary of State John Kerry had watched the video. Sotloff’s current status is unknown.

The White House was compelled to acknowledge on Wednesday night that Obama had authorized an air and ground operation in Syria this summer, which failed to rescue several American hostages held by the Islamic State.

The attempted rescue represents the first known such mission by US forces since the Syrian war began more than three years ago.

Leaders in Congress, as well as the Foley family, had been made aware of the operation at the time.

“This operation involved air and ground components and was focused on a particular captor network within ISIL,” Pentagon press secretary R.-Adm. John Kirby said on Wednesday.

“Unfortunately, the mission was not successful because the hostages were not present at the targeted location.”

The Pentagon did not specify the number of hostages suspected to be held in Syria, where specifically the operation occurred or how many US military personnel were involved.

US troops were fired upon with significant force by Islamist gunmen during the operation, according to officials, resulting in no deaths but the wounding of at least one American officer.

The White House planned to keep the mission secret, one spokesman said on Wednesday night. Only when details of the operation were leaked did the White House decide to comment.

“We never intended to disclose this operation,” National Security Council spokeswoman Caitlin Hayden said, responding to media requests. “An overriding concern for the safety of the hostages and for operational security made it imperative that we preserve as much secrecy as possible.

“We only went public today when it was clear a number of media outlets were preparing to report on the operation and that we would have no choice but to acknowledge it,” Hayden said.

US Marine Corps Gen. John R. Allen (ret.) on Wednesday night lauded Obama for his actions against the Islamic State – but called on the president to broaden the mission, and to destroy the group “quickly” before its terrorism can further metastasize.

“Make no mistake,” Allen wrote, in an op-ed featured on DefenseOne, “the abomination of [the Islamic State] is a clear and present danger to the US.”

Allen, a retired four-star general who ran operations for the US in Afghanistan, was nominated in 2013 for the position of NATO’s supreme allied commander in Europe.

“The only question really is whether the US and its allies and partners will act decisively now, while they can still shape events to destroy [Islamic State],” he wrote.

Foley’s killer, who speaks in the video with a London accent, is understood to be a British citizen by the UK intelligence community, according to Prime Minister David Cameron.

Cameron called the act nothing short of “murder without any justification,” and said the perpetrator was “likely” British.

British media outlets circulated rumors of the murderer’s origins, citing sources that identified him as “John,” a member of a group of extremist Arab Londoners. Independently, government officials confirmed that close to a quarter of roughly 2,000 foreign nationals fighting in Syria are Britons.

In a statement from Massachusetts earlier in the day, Obama sternly condemned the videotaped murder of Foley.

Obama administration officials said the president reserves all options to protect Americans under threat in the region. But the president has ruled out “boots on the ground” to fight the Islamic State, either in Iraq or Syria, where the group has control over territory.

On Wednesday, Obama called the Islamic State a cancer that the world must prevent from spreading.

In a prepared statement, Lisa Monaco, assistant to the president on homeland security and counterterrorism, said the White House would be unable to confirm further details of the rescue mission for the protection of military personnel in the region.

“Earlier this summer the president authorized an operation to attempt the rescue of American citizens who were kidnapped and held by ISIL against their will in Syria,” Monaco said. “The president authorized action at this time because it was the national security team’s assessment that these hostages were in danger with each passing day in ISIL custody.

“The US government had what we believed was sufficient intelligence, and when the opportunity presented itself, the president authorized the Department of Defense to move aggressively to recover our citizens,” she said.

Meanwhile, Tehran has conflated its willingness to help curb the threat of the Islamic State with the West’s willingness to ease sanctions on Iran.

“If we agree to do something in Iraq, the other side in the negotiations will need to do something in return,” Iranian Foreign Minister Mohammad Javad Zarif said in remarks on Wednesday carried by the state-run news agency. “All the sanctions imposed on Iran over its nuclear activities should be lifted in return for its help in Iraq.”

The White House said that the two issues are not related, and that they will not be discussed jointly at the negotiating table over Iran’s nuclear work.

World powers seek to reach a comprehensive agreement ending the nuclear crisis by November 24.